1. Field of the Invention
This invention relates to methods for increasing the rate of production of methane from a subterranean coal formation by chemically stimulating the formation with an aqueous oxidizing solution to increase the production rate of methane from the formation.
2. Brief Description of the Prior Art
Substantial quantities of methane gas are found in subterranean coal formations. A variety of processes have been used in attempts to recover the methane from such coal formations more efficiently.
The simplest process is the pressure reduction process wherein a borehole is drilled into a coal formation from the surface and methane is withdrawn from the borehole by reducing the pressure to cause methane to be desorbed from and flow from the coal formation into the borehole and to the surface. This method is not efficient because coal formations are generally not extremely porous and the majority of the methane is generally not found in the pores of the coal formation but is absorbed or adsorbed to the coal. While methane can be produced from coal formations by this process, the production of methane is relatively slow.
In some coal formations, the natural permeability is sufficient to allow the removal of in situ water to permit the enhanced recovery of methane. In such formations, cleat systems developed during the coal bed diagenesis provide channel ways through which water and methane migrate to the production wells for removal. This removal of water or "de-watering" of the coal formations removes water from the channel ways and permits the flow of methane through the channel ways and to a production well at a greater rate.
Many coal formations do not have extensively developed cleat systems or have cleat systems which are not fully developed. These coal formations have very low permeability to water and gas and do not yield water or gas at significant rates. As a result, the water fills the channels, and the recovery of methane from such coal formations is difficult or impossible at significant rates. Such low permeability watercontaining coal formations may be either water saturated or less than fully water saturated. It appears that coal formations with better developed cleat systems may have been exposed to a diffusive oxidizing fluid of some type during the geologic past whereas coal formations with less developed cleat systems do not show evidence of exposure to an oxidizing fluid in the past.
The terms "absorbed" and "adsorbed" are used interchangeably in the discussion herein to refer to methane or other light hydrocarbons which are retained in or on the surfaces of carbonaceous or other materials.
Accordingly, continuing efforts have been directed to the development of methods for replicating the effects of the conditions in the better developed cleat system coal formations and increasing the production rate of methane from such formations.